In the packaging of perishable food products, a container is required that is rigid enough to retain its shape when subjected to internal pressure produced in the container while tightly sealing the food product to protect against deterioration, leakage and contamination. Thus, perishable food products are often packaged in composite tubular containers that are sealed at both ends. The tubular body portion of these containers conventionally includes three separate plies or sets of plies.
First, at least one structural body ply made of paperboard material is included, which is formed into a tubular structure by wrapping a continuous strip of body ply paperboard material around a shaping mandrel. The body ply strip may be spirally wound around the mandrel or passed through a series of forming elements so as to be wrapped in a convolute shape around the mandrel. The tube is then cut into predetermined lengths at the downstream end of the mandrel and fitted with end caps to complete the container.
Second, these containers typically include a liquid impermeable liner ply adhered to the inner surface of the paperboard body ply. The liner ply seals the food product, such as potato crisps, chips or juice, within the container, and also prevents liquids, which may possibly contaminate the food product, from entering the container. Some liner plies are also gas impermeable, so as to not only prevent food product odors from escaping the can, but also prevent atmospheric air from entering the container and spoiling the food product. Thus, while the purpose of the body ply is to provide necessary structural properties to the container, the liner ply provides barrier properties necessary to protect and maintain the perishable food product.
Finally, a label ply, which is typically a paper-based ply, is included and adhered to the outer surface of the paperboard body ply. The label ply serves two primary functions. First, a composite can label is a source of information. The label carries the graphical matter that conveys product information, instructions, and regulatory compliance information. The label is also preferably decorative and aesthetically pleasing to the consumer, which enhances shelf appeal and increases consumer interest in the food product. In particular, the gloss of a label is important for consumer appeal, with high and even gloss being preferable.
The graphics included on conventional paper labels are either rotogravure or flexographically printed and, to protect the inks used in the label printing process, may require thermosetting-type overlacquers over the inks. However, the overlacquers may not absorb evenly into the paper and thus create labels that have inconsistent gloss characteristics. In addition, the gloss levels attainable with paper labels are limited because of the surface roughness characteristics of the paper.
The second function of the label ply is to provide some physical characteristics to the composite can. The coefficient of friction of the label ply is important because the lower the coefficient of friction, the easier the composite container is handled. The container is often routed along conveyor lines and chutes as it manufactured, filled and then shipped. Thus, a composite container with a high coefficient of friction can create inefficiencies in product packaging and increase the cost of packaging the food product, while a container with a low coefficient of friction can minimize such inefficiencies and costs.
Also, the wet strength of the label ply can be an important factor in composite container design. If the container is exposed to moisture and the paperboard body ply gets wet, the strength of the can is compromised. Such moisture is sometimes encountered during shipping. At other times, condensation may form on the surface of the can if the can is refrigerated or otherwise exposed to cold temperatures and then moved to a warmer, humid environment. If the wet strength of the label ply is low, then the label will provide little additional relief to the body ply from premature rupturing of the container.
The wet strength of conventional paper label plies is especially important when the plies are used in conjunction with composite cans for dough. Composite dough cans are designed differently from other composite containers such that they can be opened by forcefully striking the container against a sharp surface, such as the edge of a kitchen counter. The edges of the body ply strip are typically abutted and held together with an unadhered or lightly adhered joint or seam. The label ply is thus designed to provide a substantial component of the total strength of the can necessary to prevent the can from prematurely opening. Accordingly, when the label ply or other outer wrapper is completely removed, the internal pressure of the dough and the force of impact against a sharp edge causes the body ply seam to separate, thus allowing the dough product to be removed from the can. As noted above, the container may be exposed to moisture which can weaken the paperboard ply. This makes the strength of the label ply even more critical and, if it is also exposed to moisture and has a low wet strength, as is the case with most paper labels, the container may prematurely rupture.
Several prior patents have mentioned the possible use of labels for dough cans or easy-open containers which are made of polymeric film. For example, U.S. Pat. No. 3,981,433 to Thornhill et al. discloses a composite container for dough with a peelable outer reinforcing or wrapper layer. According to the '433 patent, the peelable outer wrapper layer can be formed of paper, metal foil, or a suitable synthetic plastic material, such as polyethylene adhesively bonded to the outer surface of a fibrous body wall layer by way of a separately applied polyvinyl alcohol adhesive. U.S. Pat. No. 4,235,341 to Martin et al. discloses a dough container having inner and outer plies over a fiber board body ply. According to one embodiment the inner ply is a coextruded polyethylene/polystyrene film. The outer ply is described as a thermoplastic paper film. U.S. Pat. No. 5,076,440 to Drummond discloses a dough container having a label which can be of plastic film. U.S. Pat. No. 5,084,284 to McDilda et al. discloses a dough container with a label that can be made of a plastic film. U.S. Pat. No. 5,326,023 to Rice et al. discusses a dough container wherein the label layer is formed of a "suitable biaxially oriented polymer film." The film discussed therein comprises a polypropylene film.
These films could theoretically provide improved wet strength for dough can labels. However, the types of films potentially usable for dough can labels are limited. The label must be fairly easily removable so that the consumer can open the container to obtain the dough product inside. Accordingly, the type of film used must be easily separable from the body ply and easily graspable by the consumer to allow removal. Also, to properly open dough cans, the label is preferably completely removed or at least completely torn along a predetermined line so that the spiral seam of the body ply can be completely separated. Accordingly, the film label must be of such a type that it can be easily and completely removed from the container. Only films which have predictable tear characteristics; i.e, those which tear in a predictable direction when pulled by a consumer, could be used for dough cans because of the difficulty in completely removing the label or exposing the body ply seam. However, films having predictable tear characteristics may not provide desired levels of gloss or be of sufficiently low cost to be produced commercially. In addition, any graphics or other information printed on the film labels as proposed by the prior dough container patents would be exposed to scratching during handling of the containers, thus making a less attractive appearance.
Accordingly, it would be desirable to provide a decorative composite container and a method for manufacturing such a container that increases the gloss and decorative aspects of the container to enhance consumer appeal. Such a container would have improved label ply wet strength compared to paper labels and a decreased coefficient of friction. The container would have graphics which are protected from scratching but which are also sufficiently decorative to attract the consumer's attention and make it more likely that the consumer will purchase the product.